A variety of overcurrent protective devices for circuit protection and isolation, including contactors, fuses and circuit breakers are currently employed for electrical distribution and control. State of the art circuit breakers include various means to limit the inrush current to predetermined values. U.S. Pat. No. 4,963,849 entitled "Compact Current Limiting Circuit Breaker", for example, describes an electronic current limiting circuit breaker that utilizes a compact slot motor and compact arc chute to minimize and control the arc when the circuit breaker contacts become separated upon intense overcurrent conditions. U.S. Pat. No. 4,970,481 entitled "Current Limiting Circuit Breaker Contact Arm Configuration" describes a current limiting circuit breaker contact arm configuration that includes an arc runner for rapidly directing the arc current into the arc chute.
Current limitation within circuit interruption devices is achieved by rapidly accelerating the movable contact arms through an open gap, and efficiently driving the arc into the arc chute wherein the short circuit current is suppressed and extinguished. Articulation of the contact arms under current limiting conditions is independent of the state of the interrupting mechanism, and results primarily from the extremely high magnetic forces generated from the high inrush current. With linear acceleration of the contact arms being dependent on mass, and rotary acceleration on inertia, it follows that a large contact arm will require more force than a small contact arm to open to a specified gap within the same period of time. This process of current limiting produces low I.sup.2 t let-through energies in low ampere rated devices with small contact arms, and high I.sup.2 t let through energies in high ampere rated devices with large contact arms. Consequently, the degree of protection afforded high ampere-rated protected circuits is limited by the size of the movable contact arms within the current limiting circuit breaker.
A method of enhancing the current limiting characteristics of high ampere rated circuit breakers is described in U.S. Pat. No. 5,539,370 entitled "Inductive Motor Protective Circuit Breaker Unit" wherein the movable contact arms are electrically connected in parallel with a positive temperature coefficient of resistance (PTCR) device and an ohmic resistor. Upon the occurrence of a short circuit overcurrent, the movable contact arms rapidly open to transfer the current into the parallel combination of the PTCR device and ohmic resistor, thereby achieving current limitation before the circuit breaker contacts become completely separated to isolate the protected circuit. Under quiescent operating conditions, the load circuit current passes primarily through the low impedance movable contact arms to thereby minimize the heat generated by the current suppressing unit. The inclusion of parallel-connected movable contact arms offers an effective means for providing current limiting in high ampere-rated circuit breakers with some additional cost and complexity.
A low cost current suppressing circuit breaker is described in U.S. patent application Ser. No. 08/797,151 entitled "Current Suppressing Circuit Breaker Unit for Inductive Motor Protection" (filed concurrently herewith). Therein, a polymer current limiting device operating in a manner described in U.S. patent application Ser. No. 08/514,076 filed 11 Aug. 1995, entitled "Current Limiting Device" is electrically connected in series with the circuit breaker contacts to rapidly suppress the short circuit current until the circuit breaker responds to isolate the protected circuit. However, under quiescent operating conditions, the polymeric material within the polymer current limiter dissipates I.sup.2 R power, which consequently limits the steady state current rating of the device.
Accordingly, it would be economically advantageous to have a low cost, high ampere-rated current limiting circuit breaker that does not exhibit such I.sup.2 R quiescent power losses.
One purpose of the invention is to provide a circuit breaker with a current limiting arc runner to rapidly suppress the short circuit current before the circuit breaker contacts isolate the protected circuit, without incurring I.sup.2 R power loss under quiescent operating conditions.